This invention relates to a magnetron, and more particularly to an improvement of the cathode structure of a magnetron.
Since magnetrons generate microwave energy at high efficiencies, they are incorporated in microwave ovens for cooking and defreezing foodstuffs. The cathode structures of the magnetrons are required to have long life, high quality and reliability.
As will be described later in more detail with reference to the accompanying drawings, a magnetron generally comprises a cathode structure disposed concentrically with a cylindrical anode. A prior art cathode structure of a magnetron comprises an upper end shield supported by a center support, a lower end shield supported by side supports, and a helical cathode filament concentric with the enter support, the upper end of the filament being secured to the upper end shield and the lower end to the lower end shield. The center support and the side supports are used as electrical lead wires for supplying heating current to the cathode filament from an external source.
Usually, the cathode filament is formed by helically winding a thorium-tungusten alloy wire having a diameter of about 0.6 mm. An assembly in which the cathode filament is connected to the upper and lower end shields is heated in an atmosphere of methane by passing electric current through the filament to form a carbide layer on the surface of the thorium-tungsten alloy wire which prevents evaporation of thorium having high emission property for thermoelectrons.
When a magnetron having a construction described above is incorporated into a microwave oven for heating and defreezing frozen foodstuffs, a relatively large filament current of, for example, 14 amperes at 3.15 volts is intermittently passed through the cathode filament for the purpose of controlling the oscillation output of the magnetron depending upon the type of the foodstuff and the method of cooking the same. For example, to defreeze frozen foodstuffs, the filament current is periodically intermitted. During the operation of a magnetron over a long time, the number of such intermittent control operations amounts to 200,000 to 300,000.
Under these operating conditions, cracks are formed in the surface of the carbide layer due to thermal stress so that the structure of the carbide layer becomes porous. This increases the area of the surface of the carbide layer and hence the heat dissipation by radiation with the result that the operating temperature of the filament decreases, thereby decreasing the number of thermionic emission from the filament. In an extreme case the magnetron becomes inoperative. For this reason, the life of the conventional cathode filament is 200,000 to 300,000 cycles when it is operated intermittently.
Moreover, the conventional cathode filament formed by helically winding a thorium-tungsten alloy wire has a small resistance against mechanical vibration and shock so that there is a fear of rupturing the relatively brittle carbide layer.
Considering the electrical characteristics, in the prior art cathode filament, since there is a relatively large gap between adjacent turns of the filament coil and accordingly there are many gaps along the axial length of the filament, the longitudinal contour of the filament becomes irregular and the irregular contour surface disturbs the electric field near the filament contour thereby rendering unstable the oscillation of the magnetron.
Moreover, the total length of coiled wire of the prior art cathode filament is comparatively large with the result that the cathode filament bears a relatively large impedance for microwaves and the microwave tends to couple with the cathode filament, thereby increasing radiation of unwanted electromagnetic waves. The large total length of thick filament results in a voluminous filament, thus requiring the filament to take long time for its warm up.